1. Field of the Invention
This invention relates to an impedance converter circuit for use in a filter, etc. More particularly, it relates to the stabilization of an impedance converter circuit employing an RC-active circuit.
2. Description of the Prior Art
The development of filters in recent years has centered on an RC-active filter which can exploit the thin-film circuit technology and the thick-film circuit technology, and the miniaturization of filters and the reduction of cost have been aimed at with the RC-active filter. However, when the thin-film and thick-film circuit technologies are utilized, the precisions of elements cannot be made high, and hence, it is a prerequisite to lower the desired precisions of circuit elements.
From the viewpoint of the low element precision, an active filter circuit employing an FDNR circuit (frequency-dependent negative resistance circuit) (FDNR type filter circuit) is advantageous. The FDNR circuit is a two-terminal circuit with one terminal grounded which realizes an impedance proportional to S.sup.-2 (where S denotes the Laplace-transformed angular frequency j.omega.) by the use of an RC-active circuit. The FDNR type filter realizes a filter equivalent to a conventional LC type filter by replacing C in the LC type filter with the FDNR circuit, L with R, and R with C.
There are various methods for materializing the FDNR circuit. One of the most typical methods is the expedient employing a GIC (general impedance converter) as published by A. Antoniou. Antonious's GIC is described in detail in literature (1) mentioned in the List of Prior Art, and the circuit arrangement thereof will be stated in detail later. The application of Antoniou's GIC to the FDNR circuit is described in detail in literature (2) mentioned in the List of Prior Art.
In a case where Antoniou's GIC used in the FDNR circuit is intended to be put into the form of a hybrid IC, interconnections and resistive elements, and further, capacitive elements are formed on an insulating substrate by the thin-film and thick-film circuit technologies, and active elements such as operational amplifiers (the expression "operational amplifier" refers to an amplifier which has differential input terminals, whose gain is sufficiently large, whose input impedance is sufficiently high and whose output impedance is sufficiently low) are arranged at predetermined positions on the insulating substrate and are connected with the various elements. The capacitive elements are sometimes formed on a stacked chip. In this case of the hybrid IC, stray capacitances appear between the interconnections formed on the insulating substrate in order to connect the various elements or between the interconnections and the earth. By way of example, this becomes equivalent to the incorporation of capacitors between the inverting inputs and non-inverting inputs of the operational amplifiers or between the inverting inputs and the earth. As a result, the hybird IC is prone to give rise to unstable operations such as the oscillation of the circuit and the fluctuation of a frequency response.